Understanding the social determinants of health (SDH) is essential to justify the central role of primary health care (PHC) as a strategy that promotes equity. SDH—including socioeconomic factors such as income, education, housing conditions, and social support—directly shape disease risk and severity, often exerting greater influence than isolated biomedical interventions. PHC stands out as a privileged setting to address these factors due to its community embeddedness, longitudinal patient relationships, and comprehensive care approach. Nevertheless, despite international guidelines recommending practices that confront social vulnerability contexts, a persistent gap remains between theory and effective implementation in everyday service delivery. This essay discusses how PHC can systematically integrate SDH into clinical and management workflows to reduce inequalities and improve population outcomes. Drawing from international evidence and innovative initiatives, it highlights the use of social screening tools, community health workers, and financing models adjusted for vulnerability, while addressing both current limitations and opportunities. The theory of "fundamental causes" explains that individuals with higher socioeconomic status possess a repertoire of resources—such as education, social capital, prestige, and power—that provide continuous protection against new diseases, perpetuating health inequalities even in biomedical advancement contexts. Consequently, improving environmental conditions alone does not eliminate disparities: privileged groups quickly adopt new preventive measures, maintaining the social gradient in health. Despite recognizing SDH's importance, most PHC professionals lack training, preparation, and institutional resources to address them effectively. Initiatives in U.S. community health centers using tools like PRAPARE demonstrate that data collection alone is insufficient without strong links to social services. High rates of food insecurity, inadequate housing, and transportation issues among PHC users compromise treatment adherence and access, creating real barriers to humanized care. To overcome a strictly biomedical model, PHC must incorporate preventive and community-based strategies guided by theoretical frameworks for screening, resource allocation, and monitoring SDH-related outcomes. Examples include "social prescribing" in the UK and the integration of "link workers" to connect patients with social support services—both shown to reduce hospitalizations and improve community well-being. Financing models that consider social vulnerability, such as capitation adjustments and specific SDH coding (e.g., ICD-10 Z55–Z65), promote resource allocation aligned with need. Tools like the Area Deprivation Index (ADI) aid in mapping vulnerability and guiding resource planning. Sustainable action on SDH requires investments in professional training—both undergraduate and continuing education—to enable health workers to identify, refer, and monitor socially driven health needs. PHC's transformative potential depends on political commitment, reoriented care models, and public policies that ensure appropriate funding for vulnerable populations, making the promotion of health and social justice a feasible mission.